Good color reproduction by a marking device requires accurate color maps in the marking device. A marking device's color reproduction errors show up in the colorimetry of its output. Colorimetry is the science of color measurement using standard models of human color perception.
A marking device may be required to use process colors to approximate input colors, such as spot colors. Spot colors are fundamentally different from process colors. Each spot color is printed with a specific colorant or ink. In contrast, process colors are combinable colors; they are overlaid or mixed to achieve a desired color. The process colors a marking device uses are called device colors.
A marking device that uses process colors may not be able to reproduce colors with good colorimetric accuracy. Input colors that are out of a marking device's color range can be referred to as “out-of-gamut.” The marking device will exhibit measured colorimetry errors when outputting either out-of-gamut or in-gamut colors.
A marking device may or may not map input colors directly to a colorimetric set or space. A marking device may use an intermediate color space to map an input color space to a colorimetric space. The marking device can map an input color space to the intermediate color space then to the colorimetric space. The intermediate color space provides a touch-stone so color reproduction can be device-independent.
The International Color Consortium (ICC) has defined and standardized a device-independent intermediate color space called the profile connection space (PCS). The ICC defines profiles that use specific data structures to standardize how color is managed by displays and marking devices. ICC profiles contain numerous smaller data strictures called tags. Some tags quantify a marking device's gamut and determine how it is used while other tags hold maps or lookup tables between color spaces.
Existing color reproduction methods use maps that contain errors. Color maps can be produced when a marking device's ICC profile is updated, such as when a printer is calibrated at the factory or at a customer location. Because of the perceptual inhomogeneity of colorimetric space, color mapping errors may be difficult to avoid using standard printer calibration methods. Errors in color maps can increase as the marking characteristics or status of a marking device shifts. Erroneous color maps can also be produced when colorimetric measurements contain error. Color mapping errors can exceed human perceptual thresholds when printing spot colors or calibrating a printer and can be objectionable.